Fission fragment distributions and their impact on the r -process nucleosynthesis in neutron star mergers

Abstract

Neutron star (NS) merger ejecta offer viable sites for the production of heavy r-process elements with nuclear mass numbers A 140. The crucial role of fission recycling is responsible for the robustness of this site against many astrophysical uncertainties. Here, we introduce improvements to our scission-point model, called SPY, to derive the fission fragment distribution for all neutron-rich fissioning nuclei of relevance in r-process calculations. These improvements include a phenomenological modification of the scission distance and a smoothing procedure of the distribution. Such corrections lead to much better agreement with experimental fission yields. Those yields are also used to estimate the number of neutrons emitted by the excited fragments on the basis of different neutron evaporation models. Our fission yields are extensively compared to those predicted by the GEF (general description of fission observables) model. The impact of fission on the r-process nucleosynthesis in binary neutron mergers is also reanalyzed. Two scenarios are considered, the first one with low initial electron fraction subject to intense fission recycling, in contrast to the second one, which includes weak interactions on nucleons. The various regions of the nuclear chart responsible for fission recycling during the neutron irradiation and after freeze-out are discussed. The contribution fission processes may have to the final abundance distribution is also studied in detail in the light of newly defined quantitative indicators describing the fission recycling, the fission seeds, and the fission progenitors. In particular, those allow us to estimate the contribution of fission to the final abundance distribution stemming from specific heavy nuclei. Calculations obtained with SPY and GEF fission fragment distributions are compared for both r-process scenarios.

Skyrme-Hartree-Fock-Bogoliubov mass models on a 3D mesh: effect of triaxial shape

“…Refs. [95][96][97]), incorporates non-standard t 4 and t 5 terms in the Skyrme functional, i.e. terms that are simultaneously momentum and density dependent.…”

“…In the case of the dynamical ejecta, the same model as described in Ref. [97] and corresponding to a symmetric 1.365-1.365M binary system obtained with the SFHo equa-Fig. 14 (Color online) Potential energy per particle E pot /A in each (S, T ) channel as indicated as a function of density for charge-symmetric infinite nuclear matter for BSkG1, BSk21 [94], BSk27 [15] and SLy4 [45,46].…”

“…Like in Ref. [97], we consider two distinct scenarios reflecting the possible impact (or not) of neutrino absorption on the initial neutron-richness of the dynamical ejecta. In scenario I, all weak interactions on free nucleons are neglected and the mean initial electron fraction at the time the temperature has dropped below 10 GK corresponds to Y e = 0.03.…”

“…Details about the nucleosynthesis calculations, additional nuclear inputs and the astrophysical scenario can be found in Ref. [97]. For both scenarios, the abundance distribution of the ejected material obtained with BSkG1 reaction rates is shown in Fig.…”

“…15 and compared to those found with the HFB-21 nuclear inputs [94], as used in Ref. [97]. Note that in both simulations the same β-decay rates as well as β-delayed neutron emission probabilities are taken from the relativistic mean-field model of Ref.…”